The present invention relates to cutting elements, particularly of the type in which a diamond layer is adhered to a carbide substrate to form a composite, and the composite is bonded to a support stud.
One type of cutting element used in rotary drilling operations in earth formations comprises an abrasive composite or compact mounted on a stud. The composite typically comprises a diamond layer adhered to a cemented carbide substrate, e.g., cemented tungsten carbide, containing a metal binder such as cobalt, and the substrate is brazed to the stud. Mounting of the cutting element in a drill bit is achieved by press-fitting or otherwise securing the stud into predrilled holes in the drill bit.
Fabrication of the composite is typically achieved by placing a cemented carbide substrate into the container of a press. A mixture of polycrystalline diamond grains and catalyst binder is placed atop the substrate and is compressed under ultra-high pressure and temperature conditions. In so doing, metal binder migrates from the substrate and "sweeps" through the diamond grains to promote a sintering of the diamond grains. As a result, the diamond grains become bonded to each other to form a diamond layer, and that diamond layer is bonded to the substrate along a planar interface. Metal binder remains disposed in the diamond layer within pores defined between the diamond grains.
A composite formed in that manner may be subject to a number of shortcomings. For example, the coefficients of thermal expansion of cemented carbide and diamond are close but not exactly the same. Thus, during heating or cooling of the composite, thermally induced stresses will occur at the interface between the diamond layer and cemented carbide substrate, the magnitude of the stresses being a function of the disparity in the thermal expansion coefficients. Another potential shortcoming which should be considered relates to the creation of internal stresses within the diamond layer which can result in a fracturing of that layer.
Those shortcomings were greatly alleviated by a cutting element disclosed in U.S. Pat. No. 4,784,023 issued to the present inventor on Nov. 15, 1988. That cutting element (depicted in the accompanying FIGS. 5 and 6) comprises a cemented carbide substrate having a surface formed with alternating ridges 2 and grooves 3. Each groove is formed by a pair of opposing side surfaces 4 interconnected by a base surface 5. When the diamond layer 6 is formed on the surface of the substrate, diamond particles will fill the grooves, whereby the final diamond layer will contain alternating ridges and grooves interlocked with the grooves and ridges of the substrate.
That cutting element alleviates the above-described shortcomings. That is, metal binder is very uniformly dispersed throughout the ridges of the diamond layer, whereby the occurrence of concentrated stresses is resisted. Also, the presence of the metal binder in the diamond layer maximizes the impact resistance of the diamond layer and provides for the attenuation of cracks resulting from back-conversion of the diamond grains. Also, the presence of the zone comprised of alternating ridges of diamond and cemented carbide serves to minimize the magnitude of thermally induced stresses between the diamond layer and the cemented carbide layer by acting as a graded stress interface.
Notwithstanding the above-described advantages achieved by the cutting element disclosed in U.S. Pat. No. 4,784,023, certain shortcomings have been observed. For example, the intersection of the side and base surfaces 4, 5 of the substrate grooves gives rise to a so-called notch effect, i.e., a tendency for cracks 7 to initiate in the vicinity of those intersections in response to the cutting loads imposed on the cutting action. Eventually, those cracks may propagate radially and result in pieces of the substrate and diamond breaking away from the cutting element. As a consequence, the useful life of the cutting element is shortened. It would be desirable to suppress the tendency for the cracks to form and propagate.